The third generation mobile communication system (IMT-2000) and the evolution system thereof can completely solve the main disadvantages of the first generation and the second generation mobile communication systems, and ensures that any terminal user with possession of mobile subscriber equipment can complete mobile communication and transmission of any message with anybody with a high quality in any mode at any time and any place around the world. Compared with the first generation and the second generation mobile communication systems, IMT-2000 is a generation of advanced mobile communication system. At present, the standards applied in IMT-2000 include: Time Division-Synchronous Code Division Multiple Access (TD-SCDMA) standard drawn by China, Code Division Multiple Access (CDMA) 2000 standard drawn by USA, and Wideband Code Division Multiple Access (WCDMA) drawn by Europe. Wherein, CDMA 2000 standard is widely used in North America and many other places in the world.
With the constant progress of internet and mobile communication technology and the constant improvement of living standard, mobile subscriber equipment are popularized rapidly. In order to meet the demand to high speed data service of terminal users with possession of mobile subscriber equipment, aiming at CDMA 2000, the evolution standards EV/DO and EV/DV of the CDMA 2000 are further developed and drawn up. Both EV/DO and EV/DV are enhanced technology based on CDMA 2000, and support wireless packet data service with higher speed than CDMA 2000. Wherein, EV/DO means that a voice service and a data service are respectively carried by two independent carrier waves, and EV/DV means that the voice service and the data service are transmitted on a same carrier wave. The mobile communication system based on EV/DO and EV/DV can provide abundant mobile multimedia services for terminal users.
An Enhanced Broadcast Multicast Service (EBCMCS) protocol is further proposed based on EV/DO. An EBCMCS system based on the EBCMCS protocol is mainly used for transmitting broadcast message to the mobile stations in the whole coverage area of a base station.
As shown in FIG. 1, the channel structure of the EBCMCS system in prior art comprises: a first unit 102 for channel encoding, a second unit 104 for channel scrambling, interleaving and duplicating, a third unit 106 for quadrature amplitude (QAM) modulation, and a fourth unit for orthogonal frequency division multiplexing (OFDM) modulation, wherein, the third unit 106 adopts 16 QAM modulation mode. The fourth unit 108 comprises: a protecting interval and pilot signal inserting module 1082, a QPSK spectrum spreading module 1084, an inverse fast Fourier transform (IFFT) module 1086, and a cycle prefix adding module 1088.
Correspondingly, the processing flow of the signal inputted into the EBCMCS system is: the signal inputted into the EBCMCS system is firstly channel encoded by the first unit, wherein, the channel encoding is ⅕ or ⅓ Turbo encoding; then the encoded signal is processed by the second unit by the means of channel scrambling, interleaving, duplicating and puncturing; and then the signal being punctured is divided into two paths of I path and Q path by the third unit in 16QAM modulation mode; and lastly, the input I path signal and Q path signal are respectively OFDM modulation processed by the fourth unit, and the processing flow is completed. Wherein, the OFDM modulation processing mode adopted by the EBCMCS system is multicarrier modulation; the QPSK spectrum spreading module arranged in the fourth unit is distinguished from the multicarrier modulation system which does not comprises the QPSK spectrum spreading module and adopts OFDM modulation processing mode, and is used for reducing the signal peak-to-average power rate (PAPR) of the multicarrier modulation system after the OFDM modulation.
Generally speaking, the signal PAPR in the multicarrier modulation system is obtained in this way: in the multicarrier modulation system adopting the OFDM modulation processing mode, provided that the input signal sequence which has N length is X=[X(0), X(1), . . . , X(N−1)]T, wherein, N is the number of OFDM sub-carriers. Provided that the duration time of the input signal X(n) is T, then each corresponding input signal X(n) is modulated onto one OFDM sub-carrier, which is to say, {fn, n=0, 1, . . . , N−1}. At this moment, the N OFDM sub-carriers are supposed to be orthogonal, and fn=nΔf, Δf=1/(NT), wherein, T is the duration time of the OFDM signal. Finally, by adopting the formula (1), the signal being OFDM modulation processed can be expressed as:
                                          x            ⁡                          (              t              )                                =                                    1                              N                                      ⁢                                          ∑                                  n                  =                  0                                                  N                  -                  1                                            ⁢                                                X                  ⁡                                      (                    n                    )                                                  ⁢                                  ⅇ                                      j2π                    ⁢                                                                                  ⁢                                          f                      n                                        ⁢                    t                                                                                      ,                  0          ≤          t          ≤          NT                                    (        1        )            
Nyquist sampling is then carried out to the signal. By adopting the formula (2), the discrete signal obtained can be expressed as:
                                          x            ⁡                          (              k              )                                =                                    1                              N                                      ⁢                                          ∑                                  n                  =                  0                                                  N                  -                  1                                            ⁢                                                X                  ⁡                                      (                    n                    )                                                  ⁢                                  ⅇ                                      j                    ⁢                                                                  2                        ⁢                        π                                            N                                        ⁢                    kn                                                                                      ,                  0          ≤          n          <          N                                    (        2        )            
According to the formula (1) and formula (2), the PAPR obtained can be expressed as:
                    PAPR        =                                            max                              0                ≤                t                <                NT                                      ⁢                                                                            x                  ⁡                                      (                    t                    )                                                                              2                                            E            ⁡                          [                                                                                      x                    ⁡                                          (                      t                      )                                                                                        2                            ]                                                          (        3        )            
Compared with single-carrier modulation system, the output of the multicarrier modulation system is a superposed signal of a plurality of sub-channel signals due to the OFDM modulation processing mode. Therefore, when the phase of the plurality of sub-channel signals is coincident, the instantaneous power of the superposed signal obtained will be much larger than the average power of the sub-channel signals, which results in comparatively big signal PAPR. As a result, high requirements are proposed to the linearity of the amplifier in a transmitter. If the linearity range of the amplifier can not meet the need of the sub-channel signal's change, the sub-channel signal will distort and the spectrum of the superposed signal will change, with the result that the orthogonality among sub-channel signals will be destroyed, mutual interference will be generated, and the multicarrier modulation system performance will be worsened.
Due to the fact that the EBCMCS system is also a multicarrier modulation system adopting the OFDM modulation processing mode, the EBCMCS system reduces the signal PAPR of the multicarrier modulation system after being OFDM modulation processed through further increasing the QPSK spectrum spreading module, however, the QPSK spectrum spreading modulation method adopted in the OFDM spectrum spreading module of the EBCMCS system in prior art is that only one path QPSK spectrum spreading modulation is performed for the input signal of the QPSK spectrum spreading module, therefore, the prior art does not have obvious signal PAPR reducing effect by adopting the QPSK spectrum spreading modulation method, it also has very high requirements to the linearity range of the power amplifier in the multicarrier modulation system, which not only is adverse to the design of the power amplifier, but also increases the cost of the power amplifier.